1. Field of the Invention
The present invention relates generally to a ceramic pressure sensor, and more particularly to a ceramic pressure sensor which has a ceramic diaphragm which is deformed in response to a pressure applied thereto, and at least one resistor whose resistance value varies according to a magnitude of deformation of the diaphragm and represents the pressure applied to the diaphragm.
2. Discussion of the Prior Art
U.S. Pat. No. 4,311,980 discloses a sensing device utilizing resistor strain gages, for measuring pressure in a combustion chamber of an internal combustion engine of a motor vehicle, or pressure in a hydraulic device or other devices or systems. This pressure sensing device uses a pressure sensing element made of a ceramic material. The ceramic sensing element includes a diaphragm portion on which a thick-film resistor circuit is formed. Generally, the ceramic pressure sensing element has a comparatively high resistance to heat, and is less likely to be affected by the temperature of a subject fluid whose pressure is to be measured. Further, the ceramic pressure sensing element has a comparatively small pressure hysteresis. Thus, the ceramic pressure sensing element assures a high degree of sensing accuracy.
Another type of pressure sensing device utilizing such an excellent ceramic pressure sensing element is shown in SAE Paper No. 860474 (1986). Described more specifically by reference to FIG. 10, a ceramic pressure sensing element 2 includes a diaphragm portion 21 having resistors (not shown) formed thereon, and a ceramic capsule 10 which is bonded to the diaphragm portion 21 by a glass sealing member 11. The sensing element 2 is held in position by an upper metal block 12 and a lower metal block 13 which has a pressure hole 14 for directing a subject fluid to the diaphragm portion 21. The sensing element 2 is sandwiched between the upper and lower blocks 12, 13 such that an O-ring 15 interposed between the lower block 13 and the diaphragm portion 21 urges the sensing element 2 against the end face of the upper block 12.
The upper and lower blocks 12, 13 are held together by a metal housing 4 such that the metal housing 4 engages the outer circumferential portions of the blocks 2, 13. An electronic circuit 16 is formed on a surface of the ceramic capsule 10 remote from the diaphragm portion 21. Lead wires 9 are connected at their one end to the electronic circuit 16, for applying power to the resistors on the diaphragm portion and feeding an output signal of the circuit 16 to an external device. The wires 9 extend through the block 12 so that the other ends of the wires are connected to the external device.
In the known pressure sensor constructed as described above, the pressure sensing element is held in position in pressed contact with the two blocks 12, 13 which are held together by the metal housing 4, there may arise thermal stresses due to a difference in coefficient of thermal expansion between the ceramic material of the sensing element and the metallic materials of the housing and blocks 4, 12, 13, where the sensing element is used to measure the pressure of a high-temperature fluid such as a gas within a combustion chamber of an internal combustion engine. In the known arrangement, no provisions are made for solving this problem of thermal stresses.
Another problem encountered on the known pressure sensor arises from the manner in which the diaphragm portion is exposed to the subject fluid, when the fluid contains a corrosive substance or soot, like combustion gases emitted by an internal combustion engine. That is, the soot is likely to plug the pressure hole, or be accumulated on the sensing element. Further, the pressure hole undergoes resonance upon transient changes in the pressure therein, whereby the diaphragm portion cannot be displaced or deformed exactly in response to the change in the pressure, leading to inaccurate measurement of the pressure of the subject fluid.
In the same known pressure sensor, the lead wires are directly connected to the ceramic pressure sensing element, to apply power to the resistors and feed the output signal to the external device. When the pressure sensor is installed at a location subject to vibrations, for example, on a motor vehicle, the lead wires tend to become disconnected from the sensing element, damaged, or become subjected to short-circuiting with each other or other members of the sensor such as the metal housing.
Further, the output signals in the form of a voltage or current signal indicative of a change in the electrical resistance values of the resistors responsive to a change in the pressure of the subject fluid should be fed from the diaphragm portion to the electronic circuit and then to the external device. The signal lines extending from the diaphragm portion should be insulated by a suitable insulating member such as a glass layer. The use of the insulating member reduces the freedom of design of the pressure sensor.
As indicated above, a ceramic pressure sensing element when applied to detect pressure at a location under severe conditions, such as pressure in a combustion chamber of an internal combustion engine, the sensing element should be designed to cope with a high temperature of the fluid, a high rate of change in the pressure, and a large magnitude of vibrations.